1. Field of the Invention
The present invention relates to a data transmission apparatus and, more particularly, to transmission of data performed with the use of a cable of, for example, a television camera system or the like by generating a detection signal based on a reference signal of a phase different by a predetermined phase angle and then correcting the signal level of such detection signal to thereby ensure exact demodulation of transmitted quadrature-modulated data with facility.
2. Description of the Related Art
The video systems inclusive of television camera systems known heretofore are classified into a type for transmitting video data in a form of analog signal and a type for transmitting video data in a form of digital signal.
In transmitting video data in a form of analog signal, a coaxial cable or the like is employed as a transmission line, wherein an AGC circuit, an equalizer circuit and so forth are employed for correction of the signal level deteriorated in the transmission.
Meanwhile in transmitting video data in a form of digital signal, the video data is sampled at a predetermined sampling rate to thereby produce a digital video signal, which is then transmitted after being converted into serial data. For the purpose of effectively utilizing the existing equipment, the coaxial cable or the like used for transmission of the analog signal is also used as a transmission line, and an adaptive equalizer circuit and so forth are employed to effectively avoid any bit error that may be caused in the transmission.
It is considered that the transmission efficiency can be enhanced by applying multivalued QAM (Quadrature Amplitude Modulation) to transmission of digital video signal using such a cable. In this case, the digital video signal transmission may be performed by means of the same structure as that of a related art radio communication apparatus based on such multivalued quadrature amplitude modulation.
FIG. 1 is a block diagram of a multivalued QAM demodulation circuit applied to a radio communication a pparatus or the like. In this demodulation circuit 1, a demodulator 2 receives a multivalued quadrature amplitude modulation signal (hereinafter referred to as QAM signal) S1 and outputs synchronous detection signals based on an I-axis and a Q-axis respectively. An analog-digital converter (A/D) 3 converts such synchronous detection signals of two paths to a digital signal, and then an IQ separator 5 separates the output signal of the analog-digital converter 3 into synchronous detection results SI and SQ based on the I-axis and the Q-axis respectively.
A transversal filter 6I corrects the synchronous detection result SI by a coefficient Ck calculated in a coefficient calculator 7I and then outputs the corrected signal. A discriminator 8I converts the output signal of the transversal filter 6I to a multivalued digital signal and outputs a demodulation result DI based on the I-axis. A subtracter 9I subtracts the demodulation result DI from the output of the transversal filter 6I to thereby produce error data ERI. And a coefficient calculator 7I calculates the coefficient Ck in such a manner that the error data ERI is converged to a value 0.
Similarly, a transversal filter 6Q corrects the synchronous detection result SQ by a coefficient Ck calculated in a coefficient calculator 7Q, and a discriminator 8Q produces a demodulation result DQ based on the Q-axis from the synchronous detection result SQ. A subtracter 9Q subtracts the demodulation result DQ from the output of the transversal filter 6Q to thereby produce error data ERQ. And a coefficient calculator 7Q calculates the coefficient Ck in such a manner that the error data ERQ is converged to a value 0.
The transversal filters 6I and 6Q for processing the synchronous detection results SI and SQ respectively in the manner described are mutually the same in circuit configuration, as shown in FIG. 2. Delay circuits Z1.about.Z4, where delay times are set to the repetition periods of the synchronous detection results SI and SQ, are connected in series, and the synchronous detection result SI or SQ is inputted to one end of such series circuit. The transversal filters 6I and 6Q supply the input signals of the delay circuits Z1.about.Z4 and the output signal of the last stage to weighting circuits M1.about.M5 each having a multiplier circuit configuration, wherein the signals are weighted by the coefficients Ck (Ck1.about.Ck5) respectively and then are added to one another in an adder 10.
Due to the above processing, the demodulation circuit 1 demodulates the QAM signal on the basis of the I-axis and the Q-axis by the technique of adaptive equalization and subsequently converts the demodulation results DQ and DI to a digital signal of one path, whereby various data transmitted wirelessly in the form of QAM signal are demodulated.
However, when a digital video signal is to be transmitted from a television camera system or the like by the application of such a demodulation circuit, there arise some difficulties in realizing exact transmission of the digital video signal.
Video signal transmission is premised on wire communication using the existing equipment inclusive of the coaxial cable and so forth. And in such wire communication, the frequency characteristic at the receiving end is widely varied in accordance with the transmission distance. Therefore, proper convergence of the coefficients is not exactly achievable by mere employment of adaptive equalization applied customarily to wire communication, and it becomes difficult to realize correct demodulation of the digital video signal.
For solving the above problems, there may be contrived a method which transmits training data instead of digital video signal during a preset period of time to thereby achieve proper convergence of the coefficient Ck with such training data. In this method, however, another problem arises with respect to deterioration of the transmission efficiency of the digital video signal. In this case also, when the frequency characteristic is widely varied due to extension of the transmission distance, proper convergence of the coefficient Ck with exact detection of the training data is supposed to be difficult. And there may occur even a worse case where the coefficient Ck is switched in a reverse direction to increase the error data to consequently cause a state of oscillation.
Further in adaptive equalization, any bit error in the demodulation result may be widely varied due to setting of the coefficient Ck to eventually bring about difficulty in properly converging the coefficient Ck because of some harmful influence of noise and so forth.